TEMPORAL CHANGES IN NITRATE STATUS OF ORCHARD SOILS WITH VARYING MANAGEMENT PRACTICES

Abstract

Two anion exchange membrane systems (Plant Root Simulator probes and Dynambio nitrate selective anion exchange membranes) were evaluated as tools to estimate temporal changes in nitrate nitrogen (NO3-N) in the topsoils of four central Washington, USA apple orchards. Various management practices and different growing conditions were studied in the four orchards. Higher NO3-N was associated with increasing amounts of fertigated N, larger irrigation events, and surface application of alfalfa hay mulch and /or dairy compost. Nitrate nitrogen adsorbed by the membranes was lower when actively growing roots were present in the vicinity of the membranes, suggesting that uptake of NO3-N by apple tree and cover crop roots decreased the concentration of NO3-N in the soil solution. Thus, in the presence of plant roots, the estimate of the amount of phytoavailable NO3-N in the soil provided by the anion exchange membranes may be ambiguous. On the other hand, if roots can be excluded from the immediate vicinity of the membranes, the NO3-N adsorbed by the membranes may estimate the phytoavailability of NO3-N in the soil solution. Because anion exchange membranes provide information about the amount of NO3-N remaining in the soil after plant roots have met their demand, the membranes may be useful for estimating relative potential for NO3-N leaching. In summary, sequential use of anion exchange membranes appears to provide valuable information about temporal changes in the supply of soil NO3-N with various orchard management practices.